Indoleamine 2, 3-dioxygenase (IDO) is an immune-modulating enzyme produced by activated macrophages and other immune-regulatory cells (can also be exploited by many tumor cells as a strategy to escape immune response). In humans, it is encoded by the gene IDOL (also known as IDO and INDO) and expressed in many tissues, including the central nervous system (CNS), epididymis, intestine, thymus, respiratory tract, spleen, pancreas, placenta, lens, and kidney, as well as in myeloid cells, such as macrophages, dendritic cells, and microglial cells. It can catalyze the first and rate-limiting step in tryptophan catabolism to N-formyl-kynurenine. Deprivation of tryptophan and its derived metabolite can cause potent immune suppression resulting in blockage of T cell growth and activation, induction of T cell apoptosis and increase of regulatory T cells (FASEB J. 1991, 5, 2516-2522). Tryptophan-kynurenine metabolic pathway has been proved to be critical for innate and adaptive immunity. Tryptophan 2, 3-dioxygenase (TDO) is an unrelated hepatic enzyme that also degrades tryptophan along the tryptophan-kynurenine metabolic pathway. In humans, it is encoded by the gene TDO2 and expressed at high levels in the liver, placenta, and brain. It can catalyze the first and rate-limiting step of tryptophan degradation along the tryptophan-kynurenine metabolic pathway and thereby regulates systemic tryptophan levels, the same reaction catalyzed by IDO1.
Numerous preclinical studies have shown this immune tolerance pathway is involved in cancer, autoimmune, infection, transplant rejection and allergy. Increased IDO activity plays an important role in cancer proliferation and metastasis. Studies have shown that IDO can lead to inactivation of tumor specific cytotoxic T lymphocytes which then lose their ability to attack cancer cells. In fact, over-expression of IDO1 is found in many human cancers including prostate, colorectal, pancreatic, cervical, gastric, ovarian, brain and lung cancer. Inhibition of IDO1 can reverse immune function suppressed by tumor, thereby producing an effective anti-tumor immunity. Because IDO1 inhibitors can activate T cells enhancing immune function, IDO1 inhibitors may have therapeutic effect in many areas/diseases including cancer drug resistance and rejection, chronic infection, HIV infection and AIDS, autoimmune diseases such as rheumatoid arthritis, immune tolerance and prevention of fetus rejection. IDO1 inhibitors may also be used to treat neuro or neuropsychiatric disorders such as depression (Protula, et al, 2005, Blood, 106:238290; Munn et al, 1998, Science 281:11913).
Preclinical and clinical studies have shown that inhibition of IDO1 can enhance immunity and improve efficacy of various chemotherapeutic agents against tumor and other diseases caused by immune-suppression (C. J. D. Austin and L. M. Rendina, Drug Discovery Today 2014, 1-9). IDO1−/− knock-out mice are viable and healthy suggesting inhibition of IDO1 will not cause severe mechanism-based toxicity.
Also TDO expression has been implicated in diseases, including cancer, schizophrenia, depression, and bipolar disorder, and TDO is detected in varieties of human cancers, including hepatocarcinoma, melanoma, and bladder cancer. TDO2 played an important role in tumors immunosuppression. Presence of TDO induces tumor tolerance in the host's immune system by depleting tryptophan levels and producing bioactive metabolites. Depletion of tryptophan reduces T cell proliferation, whereas TDO-derived kynurenines suppress antitumor immune responses and promote tumor cell survival and motility. Systemic blockade by a TDO2 inhibitor restores the ability of mice to reject TDO expressing tumors. These studies suggest a substantial role for TDO2 in immune tolerance and tumor progression, and inhibition of TDO can reactivate the immune system to overcome tumor-induced immune resistance (Nature, 2011, 478, 197-203). Moreover, alterations in TDO function have been associated with the pathogenesis of schizophrenia and affective disorders, this pathway is now considered as a therapeutic target in cognitive diseases like bipolar disorder and neurodegenerative disorder like Alzheimer, motor neuron disease like Multiple sclerosis, Huntington or Parkinson's disease (J. Neurosci. 2007, 27, 12884-12892, Stone T W, 2013, Br J of Pharmacol, 169(6): 1211-27).
There are now efforts in developing IDO1 and/or TDO2 small molecule inhibitors to treat and prevent IDO1/TDO2 related diseases.
Studies have shown that IDO and/or TDO inhibitors are effective in treatment and prevention of immune suppression, tumor suppression, and chronic infection, viral infection including HIV infection, autoimmune diseases and fetus rejection. Inhibition of catabolism of tryptophan represents a good therapeutic approach.